Indirectly heated cathode structure



y 1952 Y E. K. SMITH 2,605,432

INDIRECTLY HEATED CATHODE STRUCTURE Filed Oct. 19, 1949 3 Sheets-Sheet l BY 7C JNZENTQR. I

MM M,

ATTORNEY July 29, 1952 E. K. SMITH 2,605,432

- INDIRECTLY HEATED CATHODE STRUCTURE Filed Oct. 19, 1949 3 Sheets-Sheet 2 HE INVEJY TOR.

L.- BY

ATTOR N EY July 29, 1952 E.- K. SMITH 5 3 INDIRECTLY HEATED CATHODE STRUCTURE Filed Oct. 19, 1949' 5 Sheets-Sheet s [.E 51 52 51 ,ELE g3 52 :f 5'0 5'2 5 45 45 H i INVENTOR.

E BY 6. 7c WAZWW,

' ATTORNEY Patented July 29, 1952 'IND'IRECTLY HEATED 'cA'rnona STRUCTURE V 'Applicationoctober 19,1949, Serial No. 122,209

, a Claims.

This invention relates to indirectly heated cathode structures for electron discharge tubes, and more particularly to cathode structures of the indirectly heated oxide coated type for gas tubes of the higher continuous current ratings. In certain types 'of electron discharge tubes, such as gas tubes capable-of conducting amperes of current,a-"relatively'large emissive cathode surface is required; and such cathode is conveniently and? .ordinarily directly' heated by comparatively' large currents at la low heater'woltage, in the orderz of three volts; In certain applications and"uses-'of"such tubes, however, as for example on airplanesgthe power supply facilities availablemake it desirable to employ a. higher voltage for heating the cathodes of the tubes,

such as for example 14' volts. The use of such a heater 'voltagefcalls for indirect heating of the cathodes I h It is desirable that the emissive surface of the cathode for -a gas tube should be heated evenly :and" uniformly, otherwise there may be localized overheating and hot spots which adversely affect the puseful --life of the cathode. When arelativelyglarge cathode emissive surface is to be'indirectlyheated, :it is desirable to provide a heating element, in such form as to radiate heat to.a -large proportion of the total cathode surface, so asto obtain the desired uniform-heating. 'Also, the larger, the'ratio of the, surfaceof the heating elementto the surface of the cathode rote, heated, the lower the temperature ,of the heating element has to be to obtain the desired cathode temperature; and it is desirable ,for long life of the heating elements and its insulation to keep its operating temperature as low as, possible; Under suchconditions, it is advantageoustoemploy a heatingelement in thexform of a distributed winding with many closely=;-,spaced1 turns --.or loops,;so' asto provide thedesired uniform heating bya-heating element" operating -at.a relatively lowtemperature. The variousturns or: loops ofsueh ,a winding for a heating element should be 'heldin. position against movementordisplacement byjar or vibration, otherwise the a desiredspace relationship for uniform heatingisdisturbed, and also the heat resistant insulation ,onhtheheater wire is likely to be damaged byarubbing contact resulting from such motion under the influence of other ,Ithings, ,the. ,heating. elementfor these v 2 v V tubes in many applicatio and usesfis not electrically isolated from the cathode, and any detericration or break-down of insulation where the heating element is in direct contact with the cathode may permit a voltage to be applied across a portion of the heating element sufiicient to burn it" out. Also, my experience indicates that, when the heater element touches the outer surface of a thin walled hollow cathode with oxide emissive coating Onjits interior, there is a tendency for the insulation of the heater to deteriorate, apparently due to some reaction between thisinsulating material and an element of the oxide coating, presumably barium, with which thethin core metal of the cathode is-impregnated; a 1 I With theseand other considerations in mind, the primary object of this invention is to'provide a simple structure for indirectlyheating a hollow cathode having an extensive einissive surface in' sucha way as to satisfy these requisites for a dependable and satisfactory life ,of the cathode'and its heating element. A further objectof the invention is to devise an indirectly heated cathode structure which is simple in design and may beeasfly'and economically produced;

. material, which is interposed between the cathode and its heat shield in-such a wayas'to have all of its parts: maintained by frictional engagement with the wall ofthe heat shield in a position close to but out of direct contact with the cathode. I 1 I I Variousother characteristic features and attributes of the cathode structure of this invention will be in part apparent, and in part pointed out as the description progresses.

Although the cathode structure of this invention may take various forms and be employed for various types of -:tubes, it is convenient in discussing the nature of-the invention to refer to a typical tube including one specific embodiment of the cathode structure of this invention, as illustrated in -the accompanying drawings. In these drawings, Fig. 1 is an elevation of the tube, partly in section; Figs. 2, 3 and 4 are transverse sections throughthe tube on the lines 2-2,

3-3 and 4-4 respectively in Fig. 1; Fig. 5 is a view of the heater winding prior to its assembly in the tube; Fig. 6 is an enlarged transverse section through the heat shielded cathode structure, taken on the line 6-6 in Fig. 7; and Fig. '7 is an enlarged view of the upper and lower parts of the heat shielded cathodel, j v

The typic'al tubestructure illustrated, including one specific form of an indirectly heated cathode embodying this invention, comprises in general an anode A, a control ela r rent, Qrgrid; G, and a heat shielded indirectly heated cathode.=

structure, all included in a glass envelope E.

The grid G, cathode K an its heat shield I-IS and heating element HE are supported-en a cir-' cular stem mount S whcih has its periphery fused to the envelope E, as indicated atl5, afterl ribs 8 (seel lig. 2);, so that the, relativelythin tantaliunsheetris,notrunduly warped or distorted whenrheatedr 1Thisahode A is supported by a rod [0, sealed in .the-uppercndofthe envelope Ein the usual maiinersiiaibm iaeai with a terminal cap (Lof well kno wnconstruction'. The lower end of this anode supporting rod I0 is welded to the centerqof the cruciform member I I, which has its arms bent, down and terminating in: flanges e ed-t theae de 'Ihe grid Ci -inthe ypi cal, tube structure shown is in the form of an inverted cup of sheet nickel having a pentrar discharge opening surrounded a c r ular ra se-d ib: LZL w ic he e d o the grid,bars l3;.are ,welded, the vertical wallof I the cupformingraskirt J 4, forthe grid to enclose" and shield zthe; region; between the ,heat shielded cathode and the discharge opening covered by the'grid barszs-szThe grid bars. 13 are preferably of tungsten formed with a, coatingto inhibit .electron emissioninthdmanner.disclosedzin the prior patent-e to D; V. EdwardsiuNo.2,012,339, August 2'7, 19355; .1: c 1

In the particularstructure illustrated, the grid G is supported by three rods 16 each bent in the form of 'a hairpin; whi-chhave their upper separated'ends welde'd to'theinsideof the skirt M of the grid G','as-best shown 'inFig. -1.- The lower 7 bent endsof these hairpin shaped rods l6 are welded t othree grid-'supportingposts (see Fig. 4), which are anchored in the circular stem mount S, one of theseposts I I1 extending through a sealin this stem-mount to the outside of the envelope to constitute a' lead'- i-n 'connection for the grid G.

The connection between each of the hairpin shapedrods lG-and its corresponding supporting post -I 1 is reinforcedby a curved brace 18- welded near its middle' te the post I and at its ends to the-rod lfi-gas best show'n'in Fig. '4,

cathode structure, which represents the subject matter of 1 this invention, comprises in 'generaIPa hollow wall cylindrical cathode K having an okide coating on its inner surface, a heating element HE; around the outside of this cathodega d multiple wallcylindrical heat shield HS surrounding and enclosing the heating ele- 1 oftl'ie cat l'ib'de cylinder illfs satiny closed by'a flanged 'tdp member'i'l welded thereto and having the usual circular discharge opening 25 therein. To this top member 24 is welded an 'ad ditionalor secondary top member 26 provided with" a similar discharge opening, but of larger outside diameter and having a peripheral flange for attachment tQ the inner can of the heat shield I 15; The iris'idle sgrface of the cathode cylinder 2!) and the bottom piece 22 of the hollow cathode thus formedgare "coated with an emmisive coating,

preferably of the barium nickelate type prepared in accordance with the prior patent to D. V. Edwards et al., No. 2,081,864, May 25, 1937,

. The heat shield I-IS iconiprises inner and outer 'cylindricallcans t28-and 29 of thinnickel which surround .the'bathOde-CK,C except for a discharge opening, and. which providefla heat shielded-enclosed space aroundzthe cathode'for the heating element HEJiIn the particular-construction ofthe heat shield-.I-IS' shown; the outer heat shield can 29 compri'seslas cylinder; :to L which is :welded the peripheral-flange. ofra bottomt3fl having two diametrically opposite iopenings for=receiving insulator: bushings 3.1. of steatite or::;like I heat resistant insulating material;- :In order. to reduce -.heat radiation from the bottom piece-'22 of thecathode through. the bottom -1 of the heat shielding structure,.a multiple wallib'ott'om'fo'r the heat shield HS..is preferably used In the arrangement shown, a plurality or thin nickel-sheets 32a,'two as shown, which are preferably'pimpled or ridged to maintain a separation between their surfaces, are spot welded together at a few pointslandto the bottom; 3210f t epater "heat shield can 29, said sheets 32a being cut away to fit "around the insulator bushi'ngs-3l. V 5

The inner heat shield can '28 comprises a'c'yli ncler which is attadhed 'at its lowere'rid by small brackets [32jt0 the bottom 30 0f the outer heat shield can 29;;Th1ee such brackets32 are shown in'Fig. 7, but other brackets not shown are used 'to' attach the lower 'e'dgj epof the inner-heat shield can 29 to the'bottomfidat a slimcie rit number of points around its periphery toprevent collapse or distortion of its relatively thin cylindrical wall, which acts as presently descr ed to hold the heater element in; the desired position, If desired, the dimensions of the insulator bushings 3| may be chosen softh'at they'will engage the inner heat shield can 28; and in that casejthe brackets 32 shown at such points maybe omitted. The upper edge of the inner heat shield" can 28 is welded to theffiang'e *of thesecondary top member '26 of the cathode K. 'Four ben't brackets 34, also welded to the flange'of the secondary'top member 26 of the cathode, have their upper ends, together with the peripheral flange of the heat shield cover 35,;w'elded tothe upper end of the outer heat shield can, said heat shield cover 35 havingla discharge opening indicated at 36 of a size comparable with the discharge opening 25 for the cathode. Four bent strips '38, constituting supporting feet for the cathode, are welded to the inside of the flange of the bottom piece 22 and have flanges at their lower ends welded to the bottom 30 of the heat shield HS A wrapping of We 12 7 thieknesses pi a sh off imr e f ribbed nickel is interposed between the inner and outer heat shield cans, as indicated at 39, to in crease the heat'shielding. The brackets 34and strips 38 are preferably made ofa width and thickness to keep down conduction of heat from the cathode to the outer heat shieldcan 29, while affording the desired'rigidity to' the assembly. f

The heating element HE of this invention comprises essentially a flat band or ribbon type winding 40 of closely spaced loops or turns of wire coated with a heat resistant insulatingmaterial. The preferred structure consists of tungsten wire bent in a zig-zag fashion as shown in Fig. 5. After the wire has been shaped, it is heated in a hydrogen atmosphere to cause the Wire to assume a substantially permanent set. This pre-, formed winding is then dipped or sprayed with aluminum oxide in a suitable binder for the desired thickness of insulation, and' is baked to form a coating or covering for the tungsten wire which has the desired heat resistant and electrical insulating qualities.

In accordance with this invention, this proformed heater winding 40 is made slightly longer, in the order of to per cent, than the inner circumference of the inner heat shield can 28; and when this winding is positioned inside this heat shield can, the inherent resiliency of this winding causes it to expand rather tightly against the inner surface of the heat shield can, thereby providing a frictional engagement with its inner wall sufficient to hold all of the parts of the winding in the proper space relation against movement under the influence of jar and vibration to which the tube may be subjected. This construction permits the loops or bends of the heater winding 40 to be spaced as closely as needed for the desired uniform heating of the cathode, without involving complex or expensive mechanicalmeans for maintaining the individual loops in place. It will be evident that the frictional engagement characteristic of this invention enables any appropriate dimension and space relationship of the heater winding to be employed, and still have all of the parts of the winding'maintained in position after assembly. Further, the heater winding 40 when in position may be closely spaced to the cylindrical wall of the cathode, but without direct contact with it. This heater winding 40 is preferably somewhat wider than the depth of the cathode K, so that the ends of the loops of this heater winding extend below the bottom piece22 of the cathode, as shown in Fig. 1, thereby enabling the heater winding 40 to radiate heat to this bottom piece 22 of the cathode and heat it to a temperature comparable with the side walls of the oathode.

For the particular tube illustrated, which is designed for operation of the heating element with fourteen volts, it is expedient to use a size and length of wire for the heater winding such that the heater voltage is applied to halves of the heater winding in multiple. In other-words, the two ends of the pre-formed heater winding 46 are connected together for one terminal-of the heating circuit, and a tap is provided at a midpoint of the winding for the other terminal of the heating circuit, as indicated by dotted lines in Fig. 5. The bared ends of the heater Winding are welded to a short connector 42 (see Figs. 6 and '7); and this connector 42 and the'bared mid-tap of the heater winding 40 are welded to angular lugs 43,

as best shown in Fig. 7; andthese lugs 43 are welded to the upper ends of rods 44 extending through the insulator bushings 3| These rods extend through seals in the stem mount S to the exterior of the envelope, as shown in Fig. 1 for one of these rods; so a's't'o constitute lead incon ,The'heat shield HS, to which the cathode K isattached as above described, 'issupportedin the envelope by three rods 45 which have their upper bent ends welded to the bottom 3|] of the outer V heat shield can 29,-and which have their lower ends anchored to the stem mount S. One of these rods 45 extends through a seal to the outside of the .envelope to constitute the cathode connection. The seal or anchorage in the stem mount S for the rods 44' supplying heating current to the heater element I-IEare preferably in the form of an enlarged bead, as best shown in'Fig. l; and a shieldingisleeve or tube 48 fits over this bead at its lowerend, and fitsat its upper end over the associated insulator bushing 3|, with a flange welded to the bottom 30 of the heat shield HS. This sleeve 48, which may be made of a suitable metal, such as nickel, or an appropriate insulating material, such as glass or steatite, shields the rods 44 connected to the heater element HE throughout their length below the heat shield HS from the gas filling in the envelope for reasonspresently explained.

In an indirectly heated cathode for gas tubes, where the heater voltage is higher than the ionizing voltage for the gas filling at its operating pressure over a moderate meanv free path distance, it is desirable that allparts of the heating circuit inside'the tube-envelope shouldbe isolated from ionization of thegas filling. Even though such portions of the heating circuit may notassume temperatures high enough for primary electron emission sufficient to initiate cumulativeionization and an arc dicharge, if these parts are exposed to the gas filling in its ionized condition otherwise initiated, the difference in potential between such parts may be suificient to maintain a discharge, with the electrons produced by ionic bombardment. Such arc discharge betweenparts of the heater circuit, even though it may be extinguished'when the alternating voltage commonly used for the heating circuit goes through zero, has the general efiect of a low resistance shortcircuit; and if this are discharge does not overheat and burn out the exposed parts, it tends to interfere with the proper operation of theheater element, and overloads the heating transformer.

For these reasons it is desirable to avoid an arc discharge between exposed parts of the heating circuit within the tube envelope; and since it is difficult, if not impracticable, to provide adequate insulation for the electrical connections to the heater windings, including the bared parts of the heater winding 40 attached to the lugs 43, and the ends of the rods 44 above the insulator' bushings 3|, all of these exposed portions of the heater element and its-circuit connections are enclosed in a space isolated from the ionized gas filling. It will be noted that the upper end of. the space around thecathode K above the heater element HE is, closed by the secondary top 26 ofthe cathodeand the region below the cathode is enclosed by the bottom 30 of the outer heat shield can 29, together with the insulator bushings 3i and the inner heat shield can 28.

Similarly, for thesame reason, the portions of the rods included in the heating circuit and reret 7g 4 l, extending'between insulatori bushings 3 l and the stem mount are shielded'by the tubes 48 from the ionized gas filling. In this connection, it should be understood that the desired isolation does not require gastight'. conne'ctions. It' is sufficient if the gap or space opening to th'e'gas filling is sufficiently. narrow and deep relative to the mean free pathof the gas filling that a sufficient number of gas-ions may not move through this opening without being deionized by surface recombination into a region around exposed electrical connections of the. heater elementand cause an arc discharge under the influence of'thefpotentials' involved andthe efiect of positive space charge. Generally speaking, ordinary spot welding between themetal surfaces, s'uc'h as the flange of the secondary top memberlfi and; the inner: heat shield canZB, will afford the desired. isolation .efi'ect, provided of course these surface'slare welded together at intervals close enough-ltd avoid wide gaps by distortion of the relatively thin'metal. In short, the desired isolation of the heater element and its circuit connections. fromthe ionized gas filling of the tube. maybe obtained by providing close surface separations that may be readily attained by ordinary fabrication and assembly procedures, without attempting to make a gas tightjoint. I

In mountingand assembling the heat shielded cathode structure above described, so as to get the heater winding 40 in position and properly electrically connected within the enclosed space between thecathode and. heat shield without damaging the insulation onthe heater winding, it is. convenient to assemble the bottom 30 of the heat shield HS, together with the shielding tubes 48 welded theret onthe rods of the stem mount S, slipping the lower ends of these tubes 48 over the enlarged beads on the stem mount around the rods 44, and welding this bottom 30 to its supporting rods 45. h The lugs 43' for the electrical connection to the heater winding 40 may then be welded to therods 44 with the insulator bushings 3| in place. The cathode, coated and fully assembled except for the secondary top member 26, may then be attached in position on the bottom 30 of the heat shield by welding the flanged ends of its supporting strips 38 to said bottom 30. The pre-formed heater winding 40 of Fig. 5, with its ends and midtap connection bared. of insulation, is bent into a circle to slipover the cathode. After placing .the heater winding 40 in this. shape around the cathode and weldingthe electrical connections belwve'enthe heater'winding andjthe lugs '43,'the inner heat shield can 28 is slid into place around theheaterwinding, which may then expand by its inherent resiliency into firm'contact and frictional engagement with the inner wall of this heat shield can 28. Thesecondary top member 26 may then be welded to the top member 24 of the cathode, and the flange of this secondarytop member 26, together with the ends of the brackets 34, are welded to the. upper end directly heated cathode. structure of this. invention'satisfies'the'various requisites described for obtaining'uniform and consistent heating of relatively large emissive surfaces at a relatively low heater operating temperature, in a simple and effective manner, and in a way to give dependable performance and acceptable life for the heater element, largely free from deterioration of insulation, burn outs, and the like. 7

It should be understood that the specific indirectly heated cathode structure illustrated and described, and in particular the particulartype of tube including such cathode structure, merely represents a typical embodiment of the invention; and various modifications, adaptations and additions may be made in the arrangement and con struction of the parts shown and described without departing from the invention.

What I claim is 1. An indirectly heated cathode structure for electron discharge tubes comprising, a hollow oxide coated cathode of cylindrical form, a heat shield around said cathode closely spaced thereto, and a heating element in the space between said cathode and heat shield, said heating element comprising closely spaced turns of heat resistant insulated wire preformed into a winding of some resiliency, said heating element when in place tending to expand against the inner wall of said heat shield, whereby the turns of said heating element are individually maintained against movement under jar and vibration largely out of contact with said cathode, by frictional engagement with said heat shield.

2. An indirectly heated cathode structure according to claim 1 in which the heating element comprises a band winding of zig-zag turns approximately ten per cent longer than the circumference of the inner Wall of the heat shield.

3. An indirectly heated cathode structure ac cording to claim 2, in which the turns of the band winding extend transversely thereof and provide substantial resiliency lengthwise of the band.

fl. An indirectly heated cathode structure for electron discharge tubes comprising, a hollow cylindrical cathode of thin core metal having an oxide coating on its interior surface, a multiple wall heat shield having an inner cylindrical wall closely spaced to said cathode, and a heating element in the space between said heat shield and cathode consisting'of a wire of a high melting temperature preformed into a band of closely spaced zig-zag turns and coated with a heat resistant insulating material, said heating element being of a width comparable with the length of said cathode and of a length slightly longer than the circumference of the inner wall of said heat shield, said heating element being contracted when assembled inside said heat shield and expanding into intimate frictional engagement with its inner wall to maintain the wire turns individually in position against jar and vibration.

5. An indirectly heated cathode structure comprising, a hollow cylindrical cathode of thin nickel coated on its interior with barium nickelate, a multiple wall. heat shield surrounding said cathode except for a discharge opening and having an inner cylindrical wall closely spaced to the side wall of. said cathode, and a heating element of closely spaced zig-zag turns of tungsten wire coated with aluminum oxide, said heating element extending around the side wall-of said cathode with its turns lengthwise of'thecathode and tending to expand against the inner wall of said heat shield, whereby s'aidiheatingv element has its turns individually maintained position close to but out of contact with the surface of said cathode by frictional engagement with said inner wall of the heat shield.

6. An indirectly heated cathode structure for gas tubes comprising, a hollow cylindrical cathode of thin core metal having an oxide coating on its inner side wall and bottom, a multiple wall heat shield inclosing said cathode and having an inner cylindrical wall closely spaced to the side wall of said cathode, a heater winding inside said heat shield in the space between its inner wall and said cathode, said heater winding having its individual turns held in position by frictional engagement with the inner wall of said heat shield, and a top member having a discharge opening therein and attached to the open end of said cathode and to said heat shield with closely spaced surfaces, whereby the space for said heating winding adjacent the open end of the oathode is. isolated from ionization of the gas filling of the tube.

7. An electron discharge tube comprising a gas filled envelope, an indirectly heated cathodestructure enclosed in said envelope, said cathode structure comprising a hollow cathode, a heat shield wholly enclosing said cathode except for a discharge opening, said heat shield including an inner wall closely spaced to the side wall of said cathode, a heater winding of insulated wire around said cathode in the space between it and the inner wall of said heat shield, leads of substantial rigidity for supplying heating current to said heater winding extending through and insulated from said heat shield and through seals in said tube envelope, said heater winding having bared portion welded to said leads inside said heat shield, said heater winding being maintained in position against movement by jar or vibration largely by frictional engagement with the inner wall of said heat shield and independently of the electrical connections to said leads, and shielding means around said leads throughout their lengths between the heat shield and the seals in the envelope for isolating these portions of said leads from ionization of the gas filling.

8. An electron discharge tube comprising a as filled envelope, an indirectly heated cathode structure enclosed in said envelope, said cathode structure comprising a hollow cylindrical cathode, a heat shield surrounding said cathode except for a discharge opening and forming .a

narrow enclosed space around the side wall of said cathode, a heater winding in said enclosed space maintained in position close to but out of contact with the side wall of said cathode largely by frictional engagement with said heating shield, leads for supplying heating current to said heater winding extending through openings in said heat shield and through seals in said envelope, electrical connections between said heater winding and said leads inside said heat shield, bushings of heat resistant insulating material around said leads and filling the openings in said heat shield, and a shielding sleeve around each lead throughout its length between said wall of said cathode, a preformed heater winding of closely spaced turns of insulated wire approximately one-tenth longer than the circumference of said inner wall of the heat shield, said heater winding being contracted when assembled in the space between said inner wall of the heat shield and the cathode and expanding into frictional engagement with said inner wall in a position close to but largely out of contact with the side wall of the cathode, a gas filled envelope enclosing said heat shield and cathode and heater winding, heating circuit connections including rods extending through and insulated from said heat shield and also extending through seals in said envelope, said heater winding having bared portions electrically connected with said rods, the parts of the heating circuit connections inside said heat shield being isolated from ionization of the gas filling by the closely spaced surfaces of the parts of the heat shield assembly, and shielding means around all of the heating circuit connections outside of the heat shield having close surface contact with said heat shield and the tube envelope for isolating these parts of the heating circuit connections from ionization of the gas filling.

10. A gas tube having a hot cathode indirectly heated by a heating element operating at a voltage higher than the ionizing voltage for the gas filling of the tube and comprising, a cylindrical cathode oxide coated on its inner surface, a heat shield formed of a plurality of spaced sheet metal walls and'surrounding said cathode except for a discharge opening, a heater winding in the space between said cathode and the inner wall of said heat shield, a pair of tubular insulators extending through holes in the bottom of said heat shield, a pair of leads connected to the terminals of the heater winding inside the heat shield and extending through said tubular insulators and seals in the'tube envelope, each of said seals for said leads including an inwardly projecting boss, and a pair of shielding sleeves one around each lead extending from the heat shield to the surpermitting movement of the heat shield relative to the tube envelope under changes in temperature without damage to the sleeves or the seals for said leads.

EARLE K. SMITH.

REFERENCES CITED The following'references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,895,361 Zecher et al Jan. 24, 1933 1,925,701 Meyer et al. Sept. 5, 1933 2,065,997 Edwards et al. Dec. 29, 1936 2,097,297 Meier Oct. 26, 1937 2,112,718 Somers Mar. 29, 1938 2,246,176 Hull June 17, 1941 2,297,721 Smith Oct. 6, 1942 2,307,979 Wisk Jan. 12, 1943 2,459,997 Edwards et al Jan. 25, 1949 2,497,911 Reilly et al. Feb. 21, 1950' 

